1. Field of the invention
This invention relates to a process of cleaning a refractory structure, in particular as a stage in the repair of damaged refractory structures.
Refractory structures of various types, such as metallurgical furnaces, coke ovens and glass melting furnaces tend to become dirty, corroded or damaged during the course of their working lives.
Damage may for example be manifest as slippage of one or more refractory blocks in relation to the main structure which results in an irregular surface profile, or as cracking of the refractory structure. It is in general desirable to re-establish the designed surface profile of the refractory structure, and it is also desirable to prevent further slippage of the block(s) in question and to fill any gap left by its or their displacement or cracking. In order to achieve these ends, it may be necessary or desirable to cut away any protruding portion of the refractory structure. Alternatively or in addition it may be necessary or desirable to cut a keyway into a slipped block and/or a neighbouring block so that a key may be formed in or inserted into the keyway to prevent further slippage. Alternatively or in addition, it may be necessary or desirable to enlarge or shape any gap left by such slippage or cracking for the formation or insertion of a suitable plug.
Damage may alternatively be due to erosion of the material of the refractory structure. Such erosion tends to impart an irregular surface profile to the structure and it is often desirable to modify that surface profile before effecting a repair to the structure.
A refractory structure may become polluted and corroded by materials which adhere thereto, for example slag, glass, mineral residues, sulphides and sulphates.
A refractory structure could of course be cleaned mechanically, for example by spraying of gas or liquid under pressure, by sand blasting, or by treatment with ultra-sound. In certain cases where the material is sublimable or combustible, one may achieve cleaning with a torch (in the case of coke ovens for example). In other cases where it is necessary to dress or rectify the surface, one may use for example using a cutting wheel, drill or other tool, but all these techniques present certain disadvantages for subsequent refractory repair. In order to clean a refractory structure or equipment and leave a surface suitable for good quality production or for subsequent repair, the operator would usually have to approach the cleaning site quite closely, and this implies that that site would have to be at a temperature which the operator could tolerate for the time necessary to effect the cleaning. This in turn implies that the refractory structure would have to be cooled from its normal operating temperature, or a temperature which is within its normal working cycle of operating temperatures. And it would have to be re-heated after cleaning and repair. In the case of industrial furnaces of various types, in order to avoid damage to the furnace as its refractory material contracts or expands, such cooling and re-heating might have to be scheduled over a period of several days or even a few weeks, and this would accordingly represent a considerable loss in production from that furnace.
2. Description of related art
A process is known from British patent specification GB 2213919-A (Glaverbel) for dressing a refractory structure, which is at an elevated temperature, wherein a comburent gas stream carrying a mixture of particles which comprises particles of one or more elements which is or are oxidisable to form one or more refractory oxides (hereinafter called "fuel particles") and refractory oxide particles, is projected against the site to be prepared and the fuel particles are caused or allowed to bum, the said mixture further incorporating a fluxing agent, such as fluorides or alkali metal salts, the fluxing action of which is such that under the heat released by combustion of the fuel particles, the refractory structure becomes softened to an extent such that the structure becomes dressed by removal or displacement of material thereof under the mechanical action of the impinging stream.
The process of GB 2213919-A is useful simply for trimming a refractory structure, or for cutting a hole therein. The process may be performed as a preliminary step in certain refractory repair processes, and particularly such repair processes as those which are themselves capable of being carried out at or near the normal operating temperature of a refractory structure.
One such repair technique has become known as ceramic welding. This type of process is illustrated by British Patent No 1,330,894 and British patent specification GB 2 170 191 A (both in the name of Glaverbel). In such ceramic welding processes, a coherent refractory mass is formed on a surface by projecting against the surface a mixture of refractory particles and fuel particles, together with oxygen. The fuel particles used are particles whose composition and granulometry are such that they react exothermically with the oxygen to result in the formation of refractory oxide and release the heat required to melt at least the surfaces of the projected refractory particles.
In the ceramic welding process as practised, a mixture of refractory particles and fuel particles (the "ceramic welding powder") is conveyed from a powder store along a feed line to a lance from which it is projected against a target surface. The gas which leaves the lance outlet with the ceramic welding powder ("the carrier gas") may be pure (commercial grade) oxygen, or it may comprise a proportion of a substantially inert gas such as nitrogen, or indeed some other gas.
We have found that when a refractory structure is treated in accordance with the teaching of GB 2213919-A, the surface of that structure is of modified composition. This is because not all of the softened material is removed from that surface, and that softened material includes material which was projected in the dressing operation. If one requires a surface to be free from foreign material, it is necessary to adopt an alternative process. In addition, fluxing agent may remain on the treated surface. Because of the presence of the fluxing agent on the surface of the refractory structure, subsequent ceramic welding may lead to a repair which is weakened and may not adhere well to the refractory structure, for example in the case of high grade refractories used at high temperature.